With an electrophotographic imaging device, various rollers are disposed in the vicinity of an electrostatic latent image support, such as a charging roller for uniformly supplying an electric charge to this electrostatic latent image support, a developing roller for supplying toner to the electrostatic latent image support, and a transfer roller for transferring the toner image from the surface of the electrostatic latent image support onto recording paper.
FIG. 1 is a diagram illustrating an example of such an electrophotographic imaging device. A charging roller 20 rotates with its surface in contact with a photosensitive element 21 (an electrostatic latent image support), and uniformly supplies an electric charge to the surface of the photosensitive element 21. A photoconductive insulator layer is formed on the surface of the photosensitive element 21, and a conductor layer is formed under this photoconductive insulator layer. An electrostatic latent image is formed when light 22 including image information strikes the surface of the photosensitive element 21. A developer 23 makes this electrostatic latent image visible.
The developer 23 comprises a toner container 25 in which toner 24 is stored, a regulator blade 26 provided to this toner container 25, a developing roller 27, a supply roller 28 for securely supporting the toner 24 on the surface of this developing roller 27, etc. The developing roller 27 comprises a conductive elastic layer concentrically provided around a conductive shaft made of SUS, an aluminum alloy, or the like, and in some cases this conductive elastic layer is covered with a protective layer (surface layer). Voltage is applied between this conductive shaft and the surface of the developing roller. The toner 24 inside the toner container 25 consists of a non-magnetic toner, is held on the surface of the developing roller 27, and is charged by contact/friction when made into a thin toner layer by the regulator blade 26. When the surface of the developing roller 27 is brought into contact with the surface of the photosensitive element 21, the thin toner layer adheres to the electrostatic latent image on the photosensitive element 21 surface, forming a toner image on the surface of the photosensitive element 21. This toner image is electrostatically adsorbed to the surface of recording paper 30 by a transfer roller 29, producing a transfer image on the recording paper 30. This transfer image is fixed to the recording paper 30 by a heating roller 31 and a pressing roller 32. A blade or other cleaner 33 is provided on the downstream side of the transfer roller 29 in order to remove any toner powder that is not transferred and remains on the surface of the photosensitive element 21. A brush or roller is sometimes used instead of this blade.
Of the various rollers used in the above-mentioned electrophotography imaging device, the charging roller, developing roller, and transfer roller all have the same basic structure. Specifically, these rollers are configured such that a conductive elastic layer is formed around a conductive shaft, and in some cases a protective layer (surface layer) covers this conductive elastic layer. This conductive elastic layer needs to allow suitable elastic deformation. For instance, in the case of a charging roller, for the charge on its surface to be supplied to the photosensitive element, there needs to be a certain contact width in the peripheral direction between the charging roller and the photosensitive element. A low-hardness roller is used for the charging roller in order to ensure the proper contact width. In the case of a developing roller, the correct contact width in the peripheral direction is required between the developing roller and the photosensitive element in order for the toner held on the surface of the developing roller to adhere efficiently to the photosensitive element. Technology has been introduced in which a low-hardness developing roller is used in order to reduce the stress on the toner and ensure the proper contact width. In the case of a transfer roller, the required reproduced image can be obtained by electrostatically transferring the toner image to a recording medium (such as recording paper), either directly or via an intermediate transfer element, but ensuring the appropriate contact width in the peripheral direction is important in order to raise the transfer efficiency, so a low-hardness roller is again used.
To achieve the desired deformation, NBR (nitrile butadiene rubber), urethane, EPDM (ethylene propylene rubber), silicone, or another such rubber is frequently used as the material for the conductive elastic layer of the various rollers discussed above. The use of low-hardness rollers has been increasing in an effort to improve the quality of reproduced images and lessen toner stress. This low hardness can be achieved by foaming the rubber layer to reduce its hardness, or by lowering the crosslink density, but a problem with such rollers is that when a printer or copy machine is started up after not being used for an extended period, strain remains in the rollers, which can lead to voids, uneven density, and so forth in the toner image or reproduced image. This happens because the rollers are fixed for an extended period in a state in which they are pressed against the photosensitive element, intermediate transfer element, or other members, so residual compression strain remains.
The inventors conducted diligent research in light of these problems, and as a result perfected a roller that has low enough hardness to ensure the proper contact width in the peripheral direction between the roller and other members even at a low pressing force, and that has little residual compression strain even when left compressed in a state in which it is pressed against another member for an extended period.